In conventional pulp mills the facilities for screening of pulp require both high capital investment and have high operating costs. Typically different components in the screening plant are connected to each other by long pipes, each piece of apparatus being independent of the other. Usually a number of centrifugal pump are utilized for pumping the fiber suspension from one station to another to ensure continuous flow and treatment. Each piece of equipment is normally designed only for one purpose, and the layout of the screen room takes up significant floor space. Typical equipment that is utilized includes knotters, such as shown in U.S. Pat. No. 4,927,529, knot washers such as shown in European published Patent Application 93890042, and pressure screens such as shown in U.S. Pat. Nos. 4,634,521, 4,776,957, 4,915,822, 4,950,402, 5,000,842, 5,147,543, and 5,172,813. The disclosures of these prior patents are hereby incorporated by reference herein as types of exemplary equipment that may be utilized in the novel and advantageous arrangement and method according to the present invention.
One of the pieces of equipment described above, namely in U.S. Pat. No. 4,634,521, removes light rejects from the pump being treated. A cylindrical screen drum is provided which rotates a vertically disposed dome-shaped cylindrical rotor. The volume inside the rotor is operatively connected with the pump treatment of reject space between the screen cylinder and the rotor. When this equipment is dismantled the rotor is found to be filled with plastic and other lightweight and unwanted material, therefore a pipe is disposed inside the rotor to extend close to the top of the rotor to allow the rejects to flow into the pipe away from the rotor.
There are some patents that teach apparatus which performs a plurality of functions. For example, pressure screens are shown in U.S. Pat. Nos. 3,677,402, 3,785,495, 3,865,243 and 3,898,157 (the disclosures which are hereby incorporated by reference herein) which disclose two sorting stages disposed within the same housing. For example U.S. Pat. No. 3,677,402 discloses an apparatus including a stationary screen cylinder and a rotor rotating inside the screen cylinder. The rotor surface is provided with openings substantially larger than the openings of the screen cylinder. Additionally, the top portion of the rotor extend above the screen cylinder. As the pulp to be treated is introduced into the apparatus it is first divided into two fractions by means of the top portion of the rotor surface which thereby rejects the larger sized particles to be discharged from the apparatus. The accept portion of the fiber suspension flows inside the rotor from where it, due to centrifugal forces, flows back through the rotor surface towards the screen cylinder so that the accept portion flows through said screen surface outside thereof and the reject portion remains inside the screen surface to be discharged from the apparatus.
Equipment is also known in which a screening unit is combined with a centrifugal cleaner.
By using some of the equipment as described above, the layout and investment costs of a screen room can be improved somewhat. However, typically such arrangements only eliminate one pump and one screen, meaning that there is still a need for a plurality of pumps with consumption of excessive floor space.
According to the present invention a fiber suspension treatment apparatus, and method of screening cellulose pulp, are provided which are extremely advantageous compared to the typical prior art as described above. The investment and operating costs of a screening plant utilizing the apparatus according to the invention can be only a fraction of what they are in the conventional art, yet there is no sacrifice in functionality.
According to one aspect of the present invention a fiber suspension treatment apparatus is provided comprising the following elements: A single pump having an inlet and an outlet. A knotter. A primary screening stage. A secondary screening stage; and the single pump outlet connected to the knotter and screening stages to provide substantially the sole motive force to a fiber suspension to move the fiber suspension through the knotter and screening stages.
Preferably the knotter and screening stages are disposed within the same housing, and define a continuous pathway for fiber suspension within the housing extending from the knotter to the primary screening stage, and then to the secondary screening stage. The knotter rejects discharge is typically connected to a knot washer external of the housing, and the accepts of the knot washer are connected to the pump inlet. A recycle conduit is also typically provided from the secondary screening stage to the pump inlet, the recycle conduit having a length of less than two meters. In fact, essentially all of the conduits utilized in the practice of the invention have a length which is less than two meters, preferably less than 1 meter.
A tertiary screening stage may be connected to the rejects outlet with the accept outlet from the tertiary screening stage is connected to the pump inlet. A single motor may be provided for simultaneously powering the knotter and the screening stages, such as by being connectable to rotatable elements of the knotter and screening stages. The knotter is typically located immediately vertically above the first screening stage, which in turn is immediately above the second screening stage. Valves are typically provided in the conduit extending from the common housing to external equipment such as the knot washer and tertiary screening stage.
According to another aspect of the present invention a fiber suspension treatment apparatus is provided comprising the following elements: A housing. A knotter disposed in the housing and having a first rotatable element. A primary screening stage disposed in the housing and having a second rotatable element. A secondary screening stage disposed in the housing and having a third rotatable element. A first accepts discharge leading from the knotter directly to the primary screening stage, and a first rejects discharge. A second accepts discharge leading from the primary screening stage directly to the secondary screening stage, and a second rejects discharge. And, a third accepts discharge from the secondary screening stage, and a third rejects discharge.
The rotatable elements are typically disposed in a straight line arrangement and are driven by a common drive, preferably a single motor disposed on a top portion of the housing and connected to the rotatable elements by drive belts. The first rejects discharge is connected to a knot washer exterior of the housing and the third rejects discharge is connected to a tertiary screening stage exterior of the housing, both by valved conduits having a length less than two meters. The primary screening stage may comprise first, second and third screening stages, the accepts from one stage leading to the next.
According to yet a further aspect of the present invention a method of screening cellulose pulp is provided which comprises the following steps: (a) pressurizing cellulose pulp to a first pressure which is higher than a discharge pressure for the pulp; (b) under substantially the sole influence of the first pressure, without repressurizing, effecting deknotting, primary screening, and secondary screening of the pulp in knotting, primary screening, and secondary screening sequential stages; and (c) positioning the stages so that each stage is less than two meters from the next stage.
There might also be the further step of discharging rejects from the knotting and passing the rejects to a knot washer substantially solely under the influence of the first pressure, without repressurizing. Also there may be the further step of discharging rejects from the primary and secondary screening stages substantially solely under the influence of the first pressure, without repressurization. Rejects may be recirculated from the secondary screening stage and the accepts from the knot washer, back to the deknotting stage substantially solely under the influence of the first pressure. Dilution liquid is also typically fed into the pulp and at least one of the deknotting and primary and secondary screening stages, and typically all of them.
It is a primary object of the present invention to minimize the capital investment and operating cost for screening of pulp. This and other objects of the inventions will become clear from an inspection of the detailed description of the invention, and from the appended claims.